题目传送门
1 /*
2 递推DP: 如果a, b, c是等差数列，且b, c, d是等差数列，那么a, b, c, d是等差数列，等比数列同理
3 判断ai-2, ai-1, ai是否是等差(比)数列，能在O(n)时间求出最长的长度
4 */
5 #include <cstdio>
6 #include <algorithm>
7 #include <cstring>
8 #include <cmath>
9 using namespace std;
10
11 typedef long long ll;
12 const int MAXN = 1e6 + 10;
13 const int INF = 0x3f3f3f3f;
14 ll a[MAXN];
15 int dp1[MAXN], dp2[MAXN];
16
17 int main(void) { //HDOJ 5328 Problem Killer
18 int T; scanf ("%d", &T);
19 while (T--) {
20 int n; scanf ("%d", &n);
21 for (int i=1; i<=n; ++i) scanf ("%I64d", &a[i]);
22 if (n <= 2) printf ("%d
", n);
23 else {
24 for (int i=1; i<=n; ++i) dp1[i] = dp2[i] = 2;
25 int ans = 2;
26 for (int i=3; i<=n; ++i) {
27 if (a[i-2] + a[i] == a[i-1] * 2) dp1[i] = dp1[i-1] + 1;
28 if (a[i-2] * a[i] == a[i-1] * a[i-1]) dp2[i] = dp2[i-1] + 1;
29 ans = max (ans, max (dp1[i], dp2[i]));
30 }
31 printf ("%d
", ans);
32 }
33 }
34
35 return 0;
36 }